Updated Kotlin to version 2.1.21.

Author: ianbrandt

gradle/libs.versions.toml

@@ -8,7 +8,7 @@ jackson-kogera = "2.19.0-beta23"
 jakarta-inject = "2.0.1"
 junit = "5.12.2"
 jvm-dependency-conflict-resolution = "2.1.2"
-kotlin = "2.1.21-RC2"
+kotlin = "2.1.21"
 kotlinx-coroutines = "1.10.2"
 kotlinx-datetime = "0.6.2"
 log4j = "2.24.1"